1. Field of the Invention
The present invention relates to a semiconductor circuit utilizing a crystalline silicon film formed on a quartz substrate or the like and, more particularly, to a semiconductor circuit having the function of an operational amplifier.
2. Description of the Related Art
Recently, research is being carried out on techniques for forming semiconductor devices utilizing a crystalline silicon film on a quartz substrate in an integrated manner. A typical example of such techniques is a technique for providing an active matrix circuit and a peripheral driving circuit for driving the same circuit on a single quartz substrate or glass substrate.
The required circuit configurations include active matrix circuits, shift register circuits and buffer circuits.
An active layer of a thin film transistor forming a part of a circuit is formed using a crystalline silicon film. A crystalline silicon film can be fabricated by forming an amorphous silicon film on a substrate and then heating it or irradiating it with laser beams or performing both to anneal it.
A thin film transistor having an active layer formed by a crystalline silicon film has better characteristics, such as mobility, than those having an active layer formed by an amorphous silicon film.
There is a need for higher levels of integration and higher performance also for circuits formed using thin film transistors.
Recently, it is contemplated to use thin film transistors to configure, on a substrate, not only logic circuits such as shift registers but also circuits having computing functions such as operational amplifiers which have conventionally been externally attached to a substrate.
Operational amplifier circuits have been generally configured using a single crystal silicon wafer.
An operational amplifier is basically comprised of a differential amplifier circuit. A differential amplifier circuit is formed by combining two transistors having similar characteristics.
In the case of a differential amplifier circuit, a change in temperature or power supply voltage affect the two transistors simultaneously. Therefore, a change in temperature or power supply voltage does not affect the output of the circuit.
In order for this to occur, the two transistors forming the differential amplifier circuit must have similar characteristics.
In practice, since it is difficult to provide two transistors having completely identical characteristics, efforts are being made toward manufacturing techniques to provide transistors that are as similar to each other as possible in their characteristics.
Thin film transistors utilizing a crystalline silicon film have mobility lower than that of MOS transistors fabricated using a single crystal silicon wafer. Further, they have a higher level of variation in characteristics.
For this reason, it has been difficult in practice to form an operational amplifier circuit using such thin film transistors.
The present invention solves this problem. Specifically, it is an object of the invention to form a practical operational amplifier circuit using thin film transistors.